1. Technical Field
The present invention relates to an X-ray computerized tomographic (CT) imaging apparatus for use with a medical examination.
2. Background Art
In recent years, there has been widely used a CT (computerized tomographic) imaging apparatus where an X-ray is radiated to a biological body under medical examination so that a quantity of transmission X ray is detected. Data are acquired based on the detected transmission X-ray quantity, and a tomographic image of the biological body is reconstructed in accordance with the acquired data.
There is also available a double-slice CT apparatus. In the double-slice CT apparatus, a detector having a fanbeam form is arranged in two rows in the direction of slice (that is in a longitudinal direction) so that the detector can simultaneously acquire two-slice worth data.
FIG. 1 is a schematic diagram showing the double-slice CT apparatus. Referring to FIG. 1, the double-slice CT apparatus includes an X-ray tube 11 for radiating X-ray to the biological body P placed within an imaging region R, detectors 141, 142 for detecting transmission X-ray quantity of the biological body P, a data acquisition unit 15 for acquiring data based on the transmission X-ray quantity detected by the detectors 141, 142, an image reconstruction unit 16 for reconstructing the tomographic image of the biological body P in accordance with the data from the data acquisition unit 15, and a display unit 17 for displaying a reconstructed tomographic image. Referring still to FIG. 1, there are also provided a wedge 12 and a slit 13, located in a close vicinity of the X-ray tube 11 and in a direction toward the detectors 141, 142, so that X-ray is uniformly exposed to the detectors 141, 142 regardless of irradiating angles, by controlling quality and dosage of the exposure X-ray.
Moreover, there is available a double-slice apparatus employed in a helical scanning system. In such a helical scanning system, a couch on which a patient is laid is continuously translated along a longitudinal direction, while X-ray beams are continuously exposed to the patient. Then, acquired are data concerning the transmission X-ray quantity from the detector and a translated amount of the couch. In accordance with the data, the tomographic images for the patient are reconstructed.
FIGS. 2A-2C illustrates arrangement and locus of the detector in the double-slice CT apparatus according to the helical scanning system. FIG. 2A shows detectors 141, 142 which rotate around a point d (rotation center) with an X-ray focal spot a in the direction b. The characteristics for detectors 141, 142 are identical to each other, and the detector 141 is disposed right next to the detector 142. FIG. 2B is a cross sectional view taken along a plane c. In FIG. 2B, there are also shown the neighboring detectors 141 and 142, and a helical locus of the detectors with respect to a slice direction, accompanied by the rotation of the detectors and the movement of a bed portion (couch).
In the conventional double-slice CT apparatus, detecting methods for detecting the transmission X-ray quantity in the two detecting system are identical to each other. Thereby, speed at which the data are acquired is twice as a case where there is provided only a single detector, and resolution for the reconstructed tomographic images is improved over the case utilizing a single detector. However, as shown in FIG. 2C, the reconstructed tomographic image contains image data based on a single energy characteristic. Unlike T.sub.1 (longitudinal magnetic relaxation time) and T.sub.2 (transverse magnetic relaxation time) modes in a magnetic resonance imaging (MRI) apparatus, the tomographic image cannot be reconstructed from plural kinds of energy characteristics in the conventional practice.